I get that this is old and hardly replied to, but since I have a very similar suggestion I'll post it in here regardless.
+1 for this, though vertical or horizontal does not matter to me.
In panel arrangement I can't drag the column browser anywhere and I really want it in the Left Main Panel. It's taking up space on the top of my playlist overview, but on the Left Main Panel I have only album cover art. It looks nice, but even when I make that panel big to increase the displayed album art size, there is a lot of empty space where the Column Browser could better be.
Much more efficient use of the space and I could add more colums to that browser then too without it looking like a squished together sandwich (I only display 2 columns in the column browser currently because A I hardly use it since I mainly work with playlists and B every column would be even tinier with more of them there. And it already looks kind of ugly, taking that space above the playlist overview. But I just can't drag it elsewhere).